1. Technological Field
The present invention relates to a cleaning method for substrate bodies and, in particular, is preferably applied as a cleaning method for a manufacturing processes of semiconductor substrate bodies, liquid crystal substrate bodies, magnetic substrate bodies, or superconducting substrate bodies.
2. Background Art
Semiconductor elements formed on semiconductor substrates have increased in density and minuteness to the sub-quarter-micron (less than 0.25 xcexcm) level. In order to obtain an increase in LSI density to the sub-quarter-micron level, it is necessary to maintain the surface of the semiconductor substrate in a state of complete ultra-cleanliness.
In other words, it is necessary to remove contaminants, such as organic materials, metals, oxides (oxide films), and the like from the surface of the semiconductor substrate, and the surface must have a flatness on the molecular order. For this reason, it is necessary to conduct the cleaning of the semiconductor substrate.
Hitherto, the cleaning method which has been employed as the method for cleaning the semiconductor substrates is commonly referred to as RCA cleaning. In such cleaning, an aqueous solution is employed which is prepared by combining, with ultrapure water, a substance having a strong removing effect with respect to organic materials, metals, microparticles, and oxides (oxide film), such as, for example, sulfuric acid, ammonia, hydrogen peroxide, hydrofluoric acid, or the like; this aqueous solution is employed at high temperatures or at room temperature.
However, it has been determined that it is impossible to completely remove inorganic materials using these chemical solutions, and furthermore, as a result of these chemical solutions, the semiconductor substrate is eroded and irregularities are formed, and it has been confirmed that this has an effect on the characteristics of the device, such as the mobility of the carriers, the insulating characteristics of the oxide film formed on the substrate, and the like.
For this reason, the development of a cleaning method has been desired which is capable, during the manufacture of highly functional semiconductor devices, of completely removing inorganic materials without causing damage to the semiconductor substrate.
Furthermore, in order to maintain the degree of cleanliness of the surfaces of the semiconductor substrate in the conventional cleaning method, the cleaning processes were complicated and lengthy, and moreover, large amounts of chemicals and ultrapure water were required, so that the facilities and the like were large in size, and this constituted an obstacle to reducing the cost of semiconductor devices.
Currently, in place of RCA cleaning, wet cleaning has been developed, which employs, as the cleaning water, hydrogenated water, in which hydrogen gas is added to ultrapure water, or ozonated water, in which ozone gas is added to ultrapure water. By controlling the pH and the oxidation-reduction potential (ORP) of the ultrapure water, it has been possible to realize a reduction in the number of processes and a reduction in the amount of chemicals and ultrapure water employed, while maintaining the cleaning effects achieved by the conventional cleaning method (Japanese Patent Application, First Publication, Number HEI 11-57636).
In wet cleaning, in order to obtain greater cleaning effects, the pH of the hydrogenated water or ozonated water which is used as the cleaning water is controlled using chemicals.
However, because the chemical concentration in the cleaning water is not constant as a result of the chemical injection method of the chemical supply apparatus, it is impossible to accurately control the pH of the cleaning water. Furthermore, the poor quality of the chemicals employed can lead to a reduction in the quality of the cleaning water. Thus the development of a method for the control of the pH and the cleaning water which is capable of easily and accurately controlling the pH of the cleaning water has been desired.
In particular, in the wet cleaning process of silicon substrates, after cleaning with a cleaning solution containing hydrofluoric acid, the substrate surface adopts a structure in which the outermost surface thereof is terminated with hydrogen molecules; this hydrogen-terminated surface maintains an electrical state which is almost identical to that within a bulk-form silicon crystal, while being a surface. For this reason, hydrogen-terminated surfaces are chemically stable surfaces.
However, it is not the case that all silicon molecules are bonded to hydrogen molecules; some silicon molecules appear on the surface in an unbonded state, and the presence of silicon molecules which are bonded to fluorine molecules has been confirmed. Such silicon molecules are extremely chemically unstable and represent likely oxidation sites.
The present invention has as an object thereof to provide a cleaning method which realizes, in the cleaning process, (1) a reduction in the number of processes, (2) a simplification of the cleaning apparatus, and (3) a reduction in the amount of chemicals and pure water employed, and which has highly superior cleaning effects and does not damage the substrate, as well as to provide a rinsing method which aids in the hydrogen termination of silicon atoms.
The present invention provides a cleaning method which is characterized in comprising:
a first process for removing organic materials and/or metallic impurities deposited on an object to be cleaned by means of an aqueous solution containing ozone;
a second process for removing microparticles deposited on a substrate while applying vibration of 500 kHz or more to an aqueous solution containing hydrogen;
a third process for removing metallic impurities and/or silicon oxide films by means of an aqueous solution containing hydrofluoric acid and hydrogen peroxide; and
a fourth process, in which vibration of 500 kHz or more is applied to an aqueous solution containing hydrogen gas, which has as an object thereof the removal of the chemical agents of processes 1 through 3 and/or the removal and prevention of redeposition of microparticles.
The cleaning process is characterized in being carried out at room temperature. In particular, in the case of cleaning processes in the manufacturing processes of semiconductor substrates, liquid crystal substrates, magnetic substrates, and superconducting substrates, it is preferable that the temperature be greater than or equal to 20xc2x0 C. and less than or equal to 30xc2x0 C.
The cleaning process using an aqueous solution containing ozone employs an aqueous solution in which ozone is dissolved in pure water or ultrapure water to an ozone concentration of 2 mg/L or more, gases other than ozone are added, and the pH thereof is regulated so as to be between 4 and 5 inclusive.
The cleaning process using an aqueous solution containing hydrogen employs an aqueous solution in which hydrogen is dissolved in pure water or ultrapure water at a concentration of 1.2 mg/L or more, and the concentration of gases other than hydrogen is regulated so as to be 0.1 mg/L or less.
In the case of cleaning processes having as the chief object thereof the removal of microparticles, it is preferable that the pH of the aqueous solution containing hydrogen be 9.0 or greater.
In the cleaning process containing hydrofluoric acid and hydrogen peroxide, an aqueous solution is employed in which the concentration of hydrofluoric acid is between 0.05 wt % and 1.0 wt % inclusive, and the concentration of hydrogen peroxide is between 0.1 wt % and 1.0 wt % inclusive.
By means of the present invention, it is possible to remove impurities, such as organic materials, metal, microparticles, and the like, which are deposited on the semiconductor substrate bodies, and this does not cause surface irregularities such as very small roughnesses in the substrate body surface. The reason for this is thought to be as follows.
First, by means of employing ultrapure water to which ozone has been added in the first process, as a result of the action of the activated oxygen, organic materials deposited on the surface of the substrate are completely oxidized and removed, and simultaneously, the surface of the substrate is mildly oxidized.
The deposited metals also enter an ionized state at this time and are dissolved; however, a portion thereof are incorporated into the oxide film.
Continuing, by means of setting the pH of the solution in the second process to one which is slightly alkaline, the electrostatic force operating between the microparticles and the object to be cleaned is made negative, and as a result of ultrasonic vibration, the microparticles may be removed from the surface of the object to be cleaned.
Next, in the third process, hydrogen peroxide is added in addition to hydrofluoric acid, and it is possible to remove the oxide film with the hydrofluoric acid while oxidizing the surface with the hydrogen peroxide, so that it is easily possible to remove impurities even if the amount of impurities is great. At this point in time, the metals incorporated in the oxide film in the first process are also removed.
Finally, in the fourth process, the chief object is the rinsing of the chemical solution used in the third process; however, in the event that microparticles are deposited, the ability is provided to remove the microparticles in addition to rinsing as a result of the effects of the hydrogen and ultrasound.
Furthermore, when the surface to be cleaned is a silicon surface, by conducting the rinsing in a reducing atmosphere in which the oxidation-reduction potential is xe2x88x920.4 V (vs. NHE), it is possible to promote a chemically stable hydrogen-terminated surface in which the atoms of the outermost surface are covered with hydrogen molecules.
In accordance with the present invention, the following effects are obtained.
The cleaning method is one in which it is possible to completely remove impurities on the substrate body surface, such as organic materials, metals, microparticles, and the like, without causing damage to the substrate body, and moreover, the processes are greatly simplified, so that it is possible to conduct the treatment in a shorter period of time, and it is possible to achieve more stable cleaning effects. Furthermore, as a result of a great reduction in the amount of chemicals and ultrapure water employed and a reduction in the manufacturing apparatus cost, it is possible to realize a reduction in cost of the final semiconductor, liquid crystal, magnetic, and superconducting materials and devices.